1. Field of the Invention
The production of salts from polycarboxylic acids with three and more carboxyl groups and cyclic amidines in organic solvents for the purpose of hardening epoxy resins to flat coatings is described in DE-AS No. 23 24 696. Thus, for example, the reaction of one mole of pyromellitic acid with 2 moles of 2-phenyl-.DELTA.2-imidazoline in solvents such as dimethyl formamide, dimethyl acetamide, N-methyl pyrrolidone, aliphatic ketones or even alcohols, leads to the disalt of the pyromellitic acid which precipitates owing to its low solubility.
In the same way, the monosalt can be obtained in the above-indicated solvents by a similarly known process by combining pyromellitic acid and 2-phenyl-.DELTA.2-imidaxoline in a mole relationship of 1:1.
Both salts, according to DE-AS No. 23 24 696, are curing agents for epoxy resins for the production of coatings with particularly valuable properties. Thus with the use of the disalt of the pyromellitic acid with 2-phenyl-.DELTA.2-imidazoline as a curing agent for epoxy resins, flat coatings with particularly good mechanical values can be obtained. The monosalt from these components leads to even lower gloss levels though the mechanical properties of the films are thereby diminished so that even relatively slight quantities of monosalt in the disalt are very negatively evident in this respect.
The use of organic solvents in the production of the salts is disadvantageous. Closed apparatus are required both for their production and their processing, toxic and flammable liquids must be processed and the solvents must be purified after the reaction by distillation so as to be able to recycle them. There have, therefore, been experiments seeking to replace the organic solvents with water. The disadvantage here, among others, is the substantially lower solubility of the 2-phenyl-.DELTA.2-imidazoline in water than in the above described solvents. The normal process is to add the pyromellitic acid to the water and, by heating to 70.degree. to 80.degree. C., to obtain a clear solution, after which the stoichiometric quantity of solid 2-phenyl-.DELTA.2-imidazoline corresponding to the mono- or disalt is obtained with stirring. After about 2 hours of stirring at about 90.degree. C. and cooling to room temperature, the precipitated salt is obtained by filtration.
In the examination of technical application of these pyromellitic salts obtained in this way, it was discovered that the least soluble monosalt exhibited values identical with the monosalt precipitated from organic solvents only when employed, as in DE-OS No. 30 26 455, in particle sizes of &lt;60 .mu.m.
In contrast with the monosalt, it was not possible to produce the disalt consistently and optimally from water. The mechanical properties of the coatings produced with epoxy resin according to DE-AS No. 23 24 696 could no longer be attained. A differential thermal analysis of the disalt produced in water indicated that it was contaminated by monosalt.
According to the production mode described above, it is thus not possible to avoid the presence of monosalt in disalt with precipitation from water. It was possible to determine further by experiment that the monosalt, as a result of its low solubility in water, allowed ony incomplete conversion into the disalt, if one mole of 2-phenyl-.DELTA.2-imidazoline were added to the slurry of one mole of monosalt in water. The reaction, even after 12 to 15 hours of boiling of these two components in water, was still not complete and, under these conditions, a partial hydrolytic splitting of the imidazoline ring must be expected.
It was found, surprisingly, that the same good mechanical properties and the same flat gloss degree of hardened epoxy resin layers could be attained if, instead of the disalt of 2-phenyl-.DELTA.2-imidazoline and pyromellitic acid precipitated from organic solvents, a monosalt of 2-phenyl-methyl-.DELTA.2-imidazoline and trimellitic acid was used as a curing agent. In the production of this salt, both organic solvents and water can be employed as a medium without disadvantage. In contrast with the disalt from 2-phenyl-.DELTA.2-imidazoline and pyromellitic acid, the monosalt from 2-phenyl-methyl-.DELTA.2-imidazoline and trimellitic acid is not hygroscopic.
The 2-phenyl-methyl-.DELTA.2-imidazoline can be produced in a known way, for example, by condensation of 1,2-propandiamine with benzonitrile, benzoic acid, or benzoic esters. In the investigation of the substances by nuclear resonance spectroscopy, it was found that what was present was not an isomer mixture but a homogeneous substance. The position of the methyl in the C.dbd.N-group, however, cannot be determined. A tautomerism is present: ##STR1##
The object of the patent under application is therefore a process for the production of flat varnishes on the basis of powder coatings of 1,2-epoxy compounds with at least one 1,2-epoxy group in the molecule and a lower melting point of &gt;40.degree. C., salts of aromatic carboxylic acids with 3 and more carboxyl groups and imidazolines as curing agents as well as the usual additives and application of the powdered varnish upon the objects to be varnished, whereby these latter are heated to temperatures of 160.degree.-240.degree. C., characterized in that the salt of 2-phenyl-.DELTA.2-imidazoline and trimellitic acid in the mole ratio of 1:1 is used as the accelerator.
As was also further surprisingly found, it is possible, without sacrificing the quality of the flat varnish manufactured in this way, to substitute 2-phenyl-.DELTA.2-imidazoline for 2-phenyl-methyl-.DELTA.2-imidazoline even up to 60 mole %.